Description of the Related Art
The genus Salmonella contains two species, Salmonella enterica, a species divided into six sub-species on the basis of biochemical characteristics and those of homologies at the DNA level, and Salmonella bongori. The genus is subdivided into more than 2000 serovarieties defined with the aid of somatic and flagellar antigens. Bacteria of the genus Salmonella are generally pathogenic for animals and for man. It is thus known that Salmonella is among the agents responsible for the most common cases of food poisoning in developed countries; that is why rapid and reliable methods for the detection of subspecies of Salmonella are important.
The salmonellae responsible for food toxi-infections belong predominantly to the subspecies I (also called group I) of S. enterica.
Toxi-infections are however not the only pathologies caused by Salmonella infections.
For example, Salmonella enterica subspecies enterica serovariety typhi (called hereinafter Typhi) is the causative agent of human typhoid fever.
Given the nature of the infections caused by salmonellae and the need especially to search for their presence in biological samples taken from patients or from foods, it appears essential to have available rapid and sensitive means for detecting their presence therein.
The standard culture methods widely used up until now for the detection of salmonellae require a substantial amount of time and are not suitable for example for monitoring the contamination of food products. In order to overcome the disadvantages of these methods, several methods based on molecular biology techniques such as hybridization tests and tests based on the polymerase chain reaction have already been proposed. Various DNA probes have been used in several hybridization and PCR procedures to detect the Salmonella subspecies in the diet. However, none of these techniques is completely satisfactory since the sequences used are not completely known or are not exclusively present in the genus Salmonella and thus can lead to cross-reactions between the probe and DNA sequences from other enterobacteria or can lead to a large number of false negatives or false positives.
The inventors have sought means allowing the specific and sensitive detection of all the salmonellae of the species S. enterica and/or S. bongori. To this end, they focused their attention on the strain Salmonella enterica subspecies enterica serovariety typhi (S. Typhi) and on the gene involved in the invasion of cells by S. Typhi.
Furthermore, they defined certain conditions allowing the specific detection of defined groups of Salmonellae, for example Group I bacteria.
It has already been shown in the prior state of the art that the Typhi strain is capable of adhering to monolayers of HeLa cells and of entering into these cells (Yabuuchi et al, 1986). However, up until now, the genetic determinants involved in this process of adhering to and entering into the cells have not been clearly identified. Elsinghorst et al (1989) have cloned a Typhi chromosomal fragment which confers on Escherichia coli type bacteria the capacity to penetrate into Henle 407 cells. Recently, another chromosomal region involved in the invasion of HeLa cells by the Typhi Ty2 strain was identified and cloned (Popoff and Dion, 1990).